Set ImageIOBase functions as static (Follow-up from "Resolve "Display Pixel type of Image in ReadImageInfo application"")

The following discussion from !179 (merged) should be addressed:

@mgrizonnet started a discussion: (+2 comments)

can we change all *AsString methods to static as suggested by @gpasero ?

@gpasero proposed to set the following function as static:

ImageIOBase::GetFileTypeAsString
ImageIOBase::GetByteOrderAsString
ImageIOBase::GetComponentTypeAsString
ImageIOBase::GetPixelTypeAsString

Edited Sep 13, 2018 by Guillaume Pasero